As transparency is required for display elements, glass substrates using glass plates have been used as substrates for the elements (JP10311987 (A)). However, for display elements using glass substrates, problems such as being heavy in weight, breakable and unbendable have been pointed out at times. Thus, use of a transparent resin film instead of a glass substrate has been proposed.
For example, polycarbonates, which have high transparency, are known as transparent resins for use in optical applications. However, their heat resistance and mechanical strength may not be sufficient to be used for manufacturing display elements. On the other hand, examples of heat resistant resins include polyimides. However, typical polyimides are brown-colored, and thus it may not be suitable for use in optical applications. As polyimides with transparency, those having an alicyclic ring structure are known. However, the problem with such polyimides is that they have poor heat resistance.
For polyamide films for use in optical applications, WO 20041039863 and JP 2008260266(A) each discloses an aromatic polyamide having diamine including a trifluoro group, which provides both high stiffness and heat resistance.
WO 2012/129422 discloses a transparent polyamide film with thermal stability and dimension stability. This transparent film is manufactured by casting a solution of aromatic polyamide and curing at a high temperature. The document discloses that the cured film has a transmittance of more than 80% over a range of 400 to 750 nm, a coefficient of thermal expansion (CTE) of less than 20 ppm/° C., and shows favorable solvent resistance. And the document discloses that the film can be used as a flexible substrate for a microelectronic device.